[UPDATE - 9/1/2011: 'Seeing reports of UUID collisions in the wild. Johannes Baagoe has a write up on why Math.random() can't be trusted. I'm not entirely sure I buy this, but I'm certainly less confident than before about the uniqueness of UUIDs generated using only Math.random().]
… as well as non-standard random IDs of arbitrary length and radix. For examples of the types of IDs it can generate, or to see performance data, check out the Math.uuid.js Test page.
Update 1/22/10: Math.uuid.js includes an Math.uuid2 Math.uuidCompact – an alternate implementation for RFC4122v4 UUIDs designed to be as compact as possible, and Math.uuidFast() – an implementation designed for performance.
Update 06/03/10: Several people have expressed concern over how random the Math.random() method is. (E.g. “If two clients load the random() lib at the same time, will they start with the same seed?”) After doing a bit of research into how various OSes handle random # generation , I’m pretty satisfied that this unlikely to be an issue. Seeding is done from a variety of sources of almost pure random numbers – mouse movement, built-in hardware support (e.g. by measuring noise in electrical circuits), various unique device IDs, BIOS checksums, memory usage statistics… and so on.